The fall-of-potential method is a representative technology for measuring earth resistance. In particular, the 3-point fall-of-potential method is usually used. In the 3-point fall-of-potential method, positioning of two auxiliary electrodes including a current electrode and a potential electrode is important during installation. In particular, the position of the potential electrode is a critical factor in accurately measuring earth resistance. The position of the potential electrode is affected by soil composition characteristics at a measuring site. With regard to this, the position of a potential electrode for providing an accurate measurement of earth resistance in a variety of soil composition types was introduced by Dawalibi [IEEE Power Engineering Society Winter Meeting, Vol. 3, pp. 23-27, 2000].